Cellular and viral RNAs which are synthesized by RNA polymerase II have a unique chemical structures at their 5' termini, referred to as the 5' cap. This structure facilitates several processes of mRNA metabolism, including splicing, transport, stability, and the initiation of translation. Because of its pivotal role in mRNA metabolism and its inherent chemical properties we are exploring the 5' cap as a therapeutic target via antisense directed technologies. Two general approaches are proposed to incapacitate an antisense specified transcript: 1) chemical alteration by hydrolysis of the phosphoanhydride linkage, and 2) structural masking by steric inhibition or competing cap specific binding motifs. Incapacitating functional groups will be attached to antisense oligonucleotides by synthesis of phosphoramidite monomers functionalized for "postconjugation" of the incapacitating motifs after automated synthesis of the oligonucleotide. Several complementary cell free biochemical assays will he utilized to assess the efficacy (including stability and mechanism of action) of these novel compounds in solution. Oligonucleotide conjugates will be assessed for their activity on therapeutically relevant transcript targets in tissue culture cell assays. Cell toxicity and uptake will also be determined in a variety of cell lines.